Celestial sightings provide information that can be used to derive platform attitude, direction and position. Celestial navigation techniques have been in use since ancient times, with stars, such as Polaris, originally providing direction information to voyagers. Until the 1950s, elevation angles were measured against a vertical reference provided by the horizon at sea. Subsequently developed mechanizations rely on inertial instruments to maintain knowledge of a local vertical when the horizon is obscured or when measurements are done from a moving platform, such as an aircraft. However, inherent drift of inertial sensors limits accuracy of the vertical measurement and, ultimately, system performance.
It has long been known possible to derive ones position by observing an orbiting satellite against a star-field background. Such a system, known as SkyMark navigation, was developed by The Charles Stark Draper Laboratory, Inc. See, for example, R. Phillips, et. al., Skymark: A Technique for Enhancing Precision Missile Navigation, Missile Sciences Conference, Monterey, Calif., Nov. 16-18, 2004. Angles between the apparent positions of an observed satellite and (assumed known) stars contain information about the location of the observer. However, in some cases, stars or satellites cannot be easily observed. For example, these observations are difficult or impossible in daylight at low elevations, without specially designed optics. In addition, during a period of time around local midnight, satellites in low earth orbit are in a shadow cast by the earth and are, therefore, not illuminated and cannot be observed.